The invention relates to an X-ray examination apparatus provided with an X-ray image intensifier tube and a television camera tube for recording the image. An electronic circuit is provided for processing the recorded image information.
Such an apparatus adapted for image subtraction has been described in U.S. Pat. No. 4,204,225. In such apparatus there is often a need to be able to provide a diagnostic image in which small contrast differences can be displayed so as to be easily recognizable. For this purpose, it is necessary to be able to amplify the video signals corresponding to these small contrast differences to a sufficient extent.
In conventional television camera tubes, with a maximum signal current of, for example, 200 nA representing brightness peaks in the image and with an amplifier having an equivalent noise current of, for example, 1 to 2 nA, with a bandwidth of, for example, 5 MHz, the signal-to-noise ratio will be from 100 to 200. For producing normal visually observable images, such a signal-to-noise ratio is admissible. However, when sufficient amplification is provided for the smaller contrast differences important for diagnostic purposes, the noise level will be inadmissible.